The invention concerns a method for the detection of gas traces in the air with the help of a photo-ionization ion mobility spectrometer, whereby a sample gas is directed into a photo-ionization chamber and the sample molecules present in the sample gas are ionized by the VUV radiation of a, lamp and whereby this ionization is amplified by adding a reagent gas.
A method of photo-ionization ion mobility spectrometry is known from U.S. Pat. No. 5,338,931.
Ion mobility spectrometers operate similarly to time-of-fright mass spectrometers, however not under a vacuum, but rather at atmospheric pressure.
Ion mobility spectrometers (IMS) are used, for example, to detect the presence of substance vapors in an environment, such as of pollutants in atmospheric air. The possibilities range from a simple alarm function at the presence of known pollutants, e.g. a chemical warfare agent, through the identification of an unknown substance, to the quantitative determination of concentration.
Typical ion mobility spectrometers have an ionization source, a reaction cell, a drift cell, e.g. in the shape of a tube, an entrance grid between the reaction and drift regions, and an ion detector. The spectrometers operate at atmospheric pressure, where the mean free path length of the gas molecules in the drift cell is small compared to its dimensions. Usually, a carrier gas, i.e. dry air, is introduced into the spectrometer together with the sample gas or vapor. The carrier gas containing the sample is fed via an inlet to the ionization source, which causes the carrier gas and sample molecules to be partially ionized. The ionization source usually consists of .sup.241 Am or .sup.63 Ni. Through collisions, the charge from the carrier gas molecules is transferred to the sample molecules, meaning that quasi-molecular ions form. There is generally an electrical potential gradient in the reaction region, so that the charged mixture is moved toward the injection grid. The latter is electrically charged and normally bars admission to the drift cell. Periodically, this potential is lowered, however, for a brief time, so that a sample ion pulse moves into the drift cell. Here, there is an approximately constant electrical drift field, i.e. a constant potential gradient which moves the ions along the axis of the cell to a detector electrode located at the end of the drift cell opposite to the end with the injection grid, collecting the charge from the ions. The arrival time of the ions relative to the pulsed opening of the injection grid is dependent upon the mobility of the ions detected. Light ions are more mobile than heavy ions and reach the detector earlier. This effect is exploited for characterization of the ions. The pulsed opening of the grid can be repeated periodically in measuring cycles in order to increase the signal-to-noise ratio by the addition of subsequently acquired ion mobility spectra, or to perform a quasi-continuous measurement.
The use of a radioactive ionization source as an emitter of .alpha. or .beta. rays limits application to the detection of those pollutants which are strongly proton affine or electronegative (basically these are only chemical warfare agents and explosives) and their use represents a certain risk especially in the nonmilitary area.
For this reason, and to expand the range of analytical applications, various efforts were undertaken how to replace the radioactive ionization sources. In U.S. Pat. No. 4,839,143 and U.S. Pat. No. 4,928,033, the use of alkali-cation emitters as ionization sources in ion mobility spectrometers is described. With these sources, ionization of the sample molecules is only possible during the positive operating mode of the ion mobility spectrometers. In this way, many electronegative pollutants evade ionization. Furthermore, considerable electrical power (in excess of one Watt) is required to heat the alkali emitter to temperatures of 600.degree.C.-800.degree. C., rapidly consuming batteries in a small battery-operated instrument.
In the international patent application WO 93/11554 A1, a corona discharge ionization source for an ion mobility spectrometers is presented. But also with this type of ionization source, electronegative pollutants are not detectable, since nitrous gases resulting from the discharge are very strongly electronegative, thus making a charge transfer to the generally weaker electronegative pollutant molecules impossible.
The application of vacuum-UV light for pollutant ionization is known from photo-ionization detectors (PID) which are used as detectors for gas chromatography or non-selective manual instruments. The VUV light is either generated by lasers, by low or high pressure lamps filled with inert gas, by constant current lamps or by flash lamps. In the above cited U.S. Pat. No. 5,338,931, a flash lamp is suggested as particularly advantageous. Although irradiation with VUV light in principle allows for the ionization of a large number of pollutant molecules, ionization is restricted to those substances with an ionization potential which is less than the energy h.nu. of the light, and which can, in addition, absorb the light. Thus for example the molecules of 1,1-dimethylhydrazine fuel can hardly be ionized in the ppm range with the light of a 10.6 eV lamp because of its low absorbency, despite its low ionization potential of 7.3 eV.
In conjunction with ion mobility spectrometers, it is known that the addition of a chemical reagent gas such as acetone or carbon tetrachloride to the carrier gas can improve the detection selectivity. The initially cited U.S. Pat. No. 5,338,931 also suggests such an addition in conjunction with photo-ionization, particularly acetone, thereby quoting two articles, i.e. Luczynski and Wincel Int. J. Mass. Spectrometry and Ion Physics, Vol. 23, pp. 37-44 (1977) and Tzeng et al. in J. Am. Soc., Vol. 111, pp. 6035-6040 (1989). It is inferred from these articles that the ionization process takes place via the formation of protonated monomers H.sup.+ (CH.sub.3).sub.2 CO and dimers H.sup.+ [(CH.sub.3).sub.2 CO].sub.2.
It is indicated in U.S. Pat. No. 5,338,931 that the addition of acetone not only increases selectivity but also detection sensitivity. It is asserted that it is not necessary for the reagent substance to have an ionization energy which is lower than the photon energy, since excited reagent molecules could be formed which are ionized through subsequent reactions. An "effective" ionization potential is therefore important. Without any further explanations, and particularly without any example, it is mentioned that the addition of a reagent gas can improve the sensitivity of an ion mobility spectrometers in the positive and negative mode.
For acetone, the only reagent substance explicitly described in U.S. Pat. No. 5,338,931, the ionization of the target substance to be detected proceeds via the intermediate formation of protonated dimers of the acetone molecule. Charge transfer to the target molecule occurs via the proton and not through direct ionization. The effectiveness of the charge transfer therefore depends for the most part upon the proton affinities of the molecules involved, not upon their ionization potential. It is absolutely necessary for the progress of the reaction that the proton affinity of the target substance is greater than that of acetone. Detection of non-proton affine target substances, such as of some hydrocarbons and chlorinated thioether, is therefore not amplified by the addition of acetone.
However, there is still a need for a method of operation for an ion mobility spectrometer with a VUV lamp as a non-radioactive ionization source for the detection of a target substance in air, in which selectivity and detection sensitivity are improved by the addition of a reagent gas which is also successful for less proton affine target substances. Preferably, by selection of the reagent gas, the method should be able to be optimized for certain target substances, or it should be possible to suppress interferents. The method should be able to be operated in the positive and negative mode if possible.